Applied Mineral Exploration
Discussion and research relevant to mineral
exploration. Fluid types and their genetic meaning for the BIF-hosted iron ores, Krivoy Rog, Ukraine.
Marta Sośnicka, Ronald J. Bakker, Curt Broman, Iain Pitcairn, Ihor Paranko & Kingsley Burlinson
Abstract
This paper
contributes to the understanding of the genesis of epigenetic,
hypogene BIF-hosted iron deposits situated in the eastern part
of Ukrainian Shield. It presents new data from the Krivoy Rog
iron mining district (Skelevatske–Magnetitove deposit, Frunze
underground mine and Balka Severnaya Krasnaya outcrop) and
focuses on the investigation of ore genesis through application
of fluid inclusion petrography, micro- thermometry, Raman
spectroscopy and baro-acoustic decrepitation of fluid
inclusions. The study investigates
inclusions preserved in quartz and magnetite associated with the
low-grade iron ores (31–37% Fe) and iron-rich quartzites (38–45%
Fe) of the Saksaganskaya Suite, as well as magnetite from the
locally named high-grade iron ores (52–56% Fe). These high-grade
ores resulted from alteration of iron quartzites in the
Saksaganskiy thrust footwall (Saksaganskiy tectonic block) and
were a precursor to supergene martite, high-grade ores (60– 70%
Fe). Based on the new data two stages of iron ore formation
(metamorphic and metasomatic) are proposed. The metamorphic
stage, resulting in formation of quartz veins within the
low-grade iron ore and iron-rich quartzites, involved fluids of
four different compositions: CO2-rich, H2O,
H2O–CO2(±N2–CH4)–NaCl(±NaHCO3)
and H2O–CO2(±N2–CH4)–NaCl.
The salinities of these fluids were relatively low (up to 7
mass% NaCl equiv.) as these fluids were derived from dehydration
and decarbonation of the BIF rocks, however the origin of the
nahcolite (NaHCO3) remains unresolved. The minimum
P–T conditions for the formation of these veins, inferred from
microthermometry are Tmin = 219–246 °C and Pmin = 130–158
MPa. The baro-acoustic decrepitation analyses of magnetite bands
indicated that the low-grade iron ore from the
Skelevatske–Magnetitove deposit was metamorphosed at T = ~530
°C.
The metasomatic
stage post-dated and partially overlapped the metamorphic stage
and led to the upgrade of iron quartzites to the high-grade iron
ores. The genesis of these ores, which are located in the
Saksaganskiy tectonic block (Saksaganskiy ore field), and the
factors controlling iron ore-forming processes are highly
controversial. According to the study of quartz-hosted fluid
inclusions from the thrust zone the metasomatic stage involved
at least three different episodes of the fluid flow,
simultaneous with thrusting and deformation. During the 1st
episode three types of fluids were introduced: CO2–CH4–N2(±
C), CO2(± N2–CH4) and
low salinity H2O–N2–CH4– NaCl
(6.38–7.1 mass% NaCl equiv.). The 2nd episode included expulsion
of the aqueous fluids H2O–N2– CH4–NaCl(±
CO2, ± C) of moderate salinities (15.22–16.76
mass% NaCl equiv.), whereas the 3rd event involved high salinity
fluids H2O–NaCl(±C) (20–35 mass% NaCl equiv.).
The fluids most probably interacted with country rocks (e.g.
schists) supplying them with CH4 and N2.
The high salinity fluids were most likely either
magmatic–hydrothermal fluids derived from the Saksaganskiy
igneous body or heated basinal brines, and they may have caused
pervasive leaching of Fe from metavolcanic and/or the BIF rocks.
The baro-acoustic decrepitation analyses of magnetite comprising
the high-grade iron ore showed formation T = ~430–500 °C.
The fluid inclusion data suggest that the upgrade to high-grade
Fe ores might be a result of the Krivoy Rog BIF alteration by
multiple flows of structurally controlled, metamorphic and
magmatic–hydrothermal fluids or heated basinal brines.
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